The need to locate vehicles, such as automobiles and boats, has been recognized for years. Various systems have been developed to monitor vehicle position. Such systems can be used by owners of vehicle fleets, such as for automobile rental and trucks involved in long distance hauling. In addition such systems can be used for detecting the locations of vehicles used by unauthorized users, e.g., stolen vehicles.
Gray et al., U.S. Pat. No. 4,651,157 discloses a system wherein the locations of vehicles being protected from theft are responsive to Loran-C emissions. Each vehicle includes a Loran-C receiver and computes the time difference between receipt at the vehicle of the Loran-C pulses. The time difference signals are transmitted from the vehicle to a central station, where computation of vehicle longitude and latitude is performed. Computing the time difference on board the vehicle is quite expensive because of the need for high frequency microprocessors and very stable clock sources. If relatively low frequency microprocessors and conventional clock sources are employed, the required accuracy of the time difference signals derived at each vehicle is not adequate to determine the location of the vehicle with a high degree of accuracy.
It is, accordingly, an object of the present invention to provide a new and improved apparatus for and method of determining the position of a vehicle having a radio receiver for low frequency electromagnetic waves, such as those derived from at least three Loran-C transmitters.
Another object of the invention is to provide a new and improved relatively inexpensive system for and method of determining the position and identity of a vehicle being used by an unauthorized user.
Another object of the invention is to provide a new and improved system for and method of determining the position of an unauthorized vehicle wherein pulses that are synchronized and correspond with received emissions from Loran-C transmitters modulate a radio wave derived on the vehicle.
Another object of the invention is to provide a new and improved system for and method of determining the position of a vehicle in response to a command signal for the vehicle identity initiated at a central master station.